As device manufacturers continue to produce smaller devices, the requirements for photomasks used in the fabrication of these devices continue to tighten. Photomasks, also known as reticles or masks, typically consist of substrates that have a patterned layer formed on the substrate. The absorber layer includes a pattern representing an image that may be transferred onto a wafer in a lithography system. As feature sizes of devices decrease, the corresponding patterns on the photomask also become smaller and more complex. Consequently, the quality of the mask has become one of the most crucial elements in establishing a robust and reliable fabrication process.
Some applications may require the use of near-wavelength structures in addition to sub-wavelength line structures. Traditionally, the sub-wavelength line structures may be fabricated using an alternating aperture phase shift mask (AAPSM). The AAPSM typically includes etched areas of the substrate that provide destructive interference, which allows lines smaller than the wavelength of light used in the lithography system to be printed on the wafer. Near-wavelength structures, however, typically are formed using a binary photomask. In some applications, such as high capacity hard disk drives, the photomask may include sub-wavelength AAPSM areas located adjacent to, or directly linked to near-wavelength areas. At the junction of the near-wavelength structure and the sub-wavelength line structure, the curvature of the feature edge may be exaggerated due to the transition between the sub-wavelength AAPSM areas and the near-wavelength AAPSM areas.
Previous techniques for reducing the curvature of a feature edge include applying optical proximity correction (OPC) or geometrically modifying etched and non-etched areas of the substrate. These techniques, however, have minimal impact on reducing the curvature of the features edges. Another technique for correcting the curvature of the feature edges includes increasing the amount of light at the point where the near-wavelength structure meets the sub-wavelength line structure through the use of intensity-attenuated, non-opaque material. This technique, however, impacts the phase-shifting nature of the etched and non-etched areas, which may reduce the quality of the sub-wavelength line structure.